The present invention relates to improved methods for annealing ferrous wire and more particularly, to improved methods for removing residual lubricants and soot formed during such annealing.
Steel wire is commonly formed by drawing through a series of dies. This drawing operation is facilitated by lubricating the wire, typically with a soap powder or other organic material. Although ferrous wire is generally annealed with residual lubricants thereon, it is necessary but quite difficult to remove such residual lubricants and soot that may be deposited on the wire during annealing. The difficulty in removing lubricants occurs from the fact that at excessive heat rates, it is difficult to readily evacuate such lubricants from the annealing furnace before cracking the lubricants. In addition, as carbon monoxide in the furnace atmosphere tends to decompose into free carbon and carbon dioxide upon cooling down from the annealing temperatures, annealed wire frequently exhibits a significant soot deposit. It has been common practice to remove resulting lubricants and soot subsequent to annealing by dipping ferrous wire, typically in large rolls from 2,000 to 20,000 lbs. each into an acidic or caustic cleaning bath. This, however, involves the consumption of considerable quantities of acid and caustic materials and the disposal of such materials when spent. In addition, labor and maintenance costs are also involved in these operations.
It is known to anneal ferrous wire under nitrogen based atmospheres as, for example, is described in an article entitled "Annealing Ferrous Wires in Nitrogen Based Atmospheres," Wire Journal International, 1983, Pages 52-55. The problems caused by contaminants in such atmospheres, such as CO.sub.2, moisture, etc. are ostensibly alleviated by introducing an additive such as hydrocarbons and/or methanol to enable the carbon potential of such atmospheres to be controlled. Most of the lubricant is evacuated from the furnace before lubricant cracking can occur. However, some residual, cracked lubricant and soot will remain on the wire upon completion of annealing. This article, however, indicates that the cooling time of a process described therein may be reduced by opening a furnace to air at a temperature of 600.degree. F. or lower or below 800.degree. F. if bluing, i.e. oxidation, may be tolerated. In addition, U.S. Pat. No. 4,016,011 indicates that air cooling of ferrous parts being heat treated under a nitrogen based atmosphere may be achieved as long as the temperature of such parts is below 800.degree. F. The problem of lubricant and soot removal is not addressed, however, in this reference.
It is also known to attempt to prevent soot formation in heat treating processes such as brazing by adding oxidants such as air, moisture, etc. into a furnace as temperature builds up. Such a process is described in an article entitled "Humidifying Furnace Atmospheres Can Prevent Soot Formation," Heat Treating, April, 1982, Pages 32 and 34-36. It is noted that soot and other residues can be formed from a reaction between the furnace atmosphere and brazing paste components, and in order to avoid leaving such residues, it is proposed to modify such paste and add air to the front end of a continuous furnace, but at a temperature greater than 1400.degree. F. to avoid explosive conditions developing in the furnace. Although this technique may be successful in certain brazing processes practiced in continuous furnaces, the same is not believed to be effective in an enclosed bell or batch furnace where vaporized contaminants, CO.sub.2, etc. will remain and decarburize ferrous parts at furnace temperatures in excess of 1100.degree. F.
U.S. Pat. No. 4,359,351 describes a process for annealing ferrous metals under protective atmospheres wherein nitrogen and methanol are utilized. Although this process may be generally satisfactory to anneal such metals, soot formation results as is described in the examples of this patent. Thus, previously known processes for annealing ferrous wire have not been successful to achieve such annealing under nitrogen based atmospheres without significant soot formation and resulting lubricant residues. In fact, lubricant removal is generally so incomplete that annealed ferrous wire must generally be dipped in a cleaning bath following annealing under nitrogen based atmospheres as generally described hereinabove.
Thus, there is a clear need for processes for annealing ferrous wire wherein lubricant removal readily occurs and soot formation is essentially precluded prior to the removal of wire from the furnace to thereby avoid the requirement to dip such wire in a cleaning bath or the like.